Stainless steel



Patented May 2, 1939 1 UNITED STATES PATENT orr cr.

and John Wulif, Cambridge, Mass., assignors to Chemical Foundation Inc., New York, N. Y., a

corporation of Delaware No Drawing. Application July 17, 1936,

Serial No. 91,096

5 Claims.

The invention described herein has to do with the iron alloys commonly known as stainless steel and with the production of a, steel which is better capable of resisting certain types of corrosive effects, particularly when immersed in sea water, and having certain other important improved characteristics, such as a greatly less work hardening effect, i. e., improvement of machinability, and an increase in heat conductivity. Our in- 1 vention provides successful means for increasing the resistance of stainless steel to the corrosive effects of sea water, and gives to the metal other valuable properties which will be explained hereafter.

The invention is particularly applicable to stainless steels of the type ofthat known'to the trade as 18-8 stainless, since it contains about 18% chromium, 8% nickel, the carbon being less than 0.35%, and to equivalent stainless steels in which the nickel is replaced, wholly or in part, by manganese. Such stainless steel strongly resists general surface corrosionand therefore is of great value when exposed to air and water under ordinary atmospheric conditions. In spite 25 of its relatively high cost, such steel is widely and successfully used in the arts.

However, it has been found that when exposed to sea water, stainless steel frequently develops hidden sub-surface pits or pockets which may be entirely invisible on the surface or are revealed only by microscopic specks, the remainder of the surface retaining its luster, although the interior may be honeycombed with destructive pits. The

formation of such pits takes place rapidly and sometimes with little or no outward sign, so that it creates an exceedingly dangerous situation.

In the following description this type of corroslon is referred to as pit corrosion or pitting to 'distinguish it from general corrosion, which takes Cir place over large surface areas which are in genera] in planes parallel to the original surface, and from intergranular corrosion, which takes place in the immediate vicinity of the grain boundaries of the metal. The troubles arising from such pitting when stainless steel has been used for marine work are so serious that doubts of thepropriety of using stainless steel where exposed to sea water have been frequently expressed. The situation is discussed in "'I'hums The Book of Stainless Steels, Cleveland, January, 1935, but since that time the situation has become much more acute owing to the failure of certain structural members such as the shaft couplings, gasoline storage tanks, pipe lines and 5 deck houses of navalvessels.

We have discovered that the addition of small quantities of silver to stainless steel containing chromium and nickel, particularly the 18-8 stainless, increases its resistance to pit corrosion and gives it other valuable properties hereinafter fully dscussed. For reasons which will be explained later, we believe our invention to be useful primarily, if not entirely, with chrome-nickel and chrome-manganese steels which do not in general contain amounts of other metals sufficient to change the general characteristics of the steel or to counteract the beneficial effects of the silver. Likewise, the invention is primarily useful with alloys in which the percentages of chromium and nickel or the equivalent approximate 18 and 8 respectively. Also we have been unable to discover any metal except silver which has all of its valuable properties when combined as described, although a combination of molybdenum and silver affords some unexpectedly favorable consequences.

The following are formula resulting from analyses of several steels embodying our invention which have been found to be particularly effective for the uses indicated, although it will be understood that each of these steels have all of the properties indicated to some degree; also that the remainder of the alloy is essentially iron, by which is not meant to exclude other sub- 30 stances, such as phosphorus, sulphur, silicon, etc., in small amounts, as is more or less usual in stainless steels of this general type.

For resistance to pit corrosion:

Formula 1 Per cent Carbon less than 0. O7 Nickel 8. 00 Chromium 18. 00 Silver -s 0. 30 Molybdenum 4. 00

Formula 2 Carbon less than -i 0. Nickel 8. Chromium 18.

Silver 0. Molybdenum 3.

Formula 3 Carbon l 0. Nickel 11. Chromium 18. Silver 0.

For free machining properties:

Formula 4 the metal. Accordingly, the addition to stainless steel of small quantities of a. metal, of which silver less g: 3 is the chief example, which forms an insoluble ;g""':':: 1,190 chloride and consequently a self-healing protec- Suver 0 15 tive film, results in almost complete protection to "T""""""''"" the body of the steel. Molybdenum has this o high Surface P property to some extent, but while lead and Formula 5 thalium also form insoluble chlorides their alloying properties do not render them useful for infi less than 16 creasing corrosion resistance of stainless steel. Nlckel Silver is completely insoluble in iron and al- Chrommm 18130 though stated to be soluble up to 10% in chromisilvel 90 um at melting temperatures, it comes out of solu- For high thermal Conductivity; tion upon solidification, and no effective amount remains in solution. On the other hand, silver Formula 6 makes a solid solution with nickel up to about Carbon o, 07 4% and with manganese up to about 20%, while Manganese 0,12 nickel and manganese are themselves soluble in Nickel 9 42 iron and chromium in all proportions. There- Chmmium 19 32 fore, although silver may be added to an ironsu 0 14 chromium alloy, none will be present in the grain Comparative total immersion tests in acidified ferric chloride solution (a very severe pitting medium for stainless steel of this general type) have shown that, in a stainless steel having a composition corresponding to Formula 3, above, the pit corrosion produced by equal periods of immersion is only about 16% of that of a standard 18- 8 stainless steel without silver.

In the following paragraphs we have stated some of our observations and our deductions from them and from the present knowledge of the behaviour of stainless steels, since these facts and deductions may contribute to a better under standing of the invention, but we do not intend to limit ourselves to any theory which we have stated although we believe them to be correct.

We have observed that pitting occurs in stainless steel exposed to the action of sea water if a portion of the surface is protected from constant exposure to the sea water. of pits will occur around the edge of a barnacle while the remaining surface of the plate will be unpittecl. Likewise, if a rubber band is placed around a stainless steel pipe and the pipe immersed in sea water, pits will be formed in a ring around the pipe adjacent the edge of the rubber band. It is reasonable to suppose that the oxygen in the air or sea water is partly excluded from the covered surface while it is freely admitted to the exposed surface and that the resulting difference in oxygen concentration produces a difference of potential which accelerates the chemical reaction and the formation of chlorides. The chlorides of iron and of chromium are readily soluble in sea water, consequently, as it is formed, the chloride goes into solution and is carried away, leaving the spot exposed for the further formation of chloride and a continuation of the electrolytic action. As the pit forms, the difference in oxygen concentration between the inside of thepit and the body of the surrounding metal is intensified and the electrolytic action is stimulated. When even minute amounts of silver are included in the alloy, the action is quite different because the chloride of silver is insoluble or only slightly soluble in sea water and is not dissolved and carried away by the sea water but remains and covers the spot, forming an insoluble protective coating over it. In this respect it behaves somewhat as does an article made of copper which, when exposed to the air, soon forms a closely adherent protective layer of copper oxide which effectively prevents further oxydation of For instance, a ring.-

upon cooling, but the added silver will be found to be segregated at the grain boundaries. This is also true of an iron-chromium-copper alloy. On

"the other hand, when nickel is present as in the 18-8 stainless steel, the silver, being soluble in the nickel-chromium-iron alloy, is in solution throughout the entire grain structure and is not merely segregated at the grain boundaries. Accordingly to obtain the beneficial effects on our invention it seems to be necessary that nickel or manganese shall be present as well as the silver to accomplish solution of the silver, and reduce the amount of segregation of silver at the grain boundaries.

These facts are believed to afford an explanation of the singular resistance of chrome-nickelsilver stainless steel to pitting when exposed to sea water. Since the entire surface of the metal the grain as well as the grain b0undarycontains silver, the water insoluble self-healing film of silver chloride forms over the entire surface of the metal, protecting it, instead of leaving the grains themselves exposed to the electrolytic efiect of the sea water if there is a difference of potential due to a difference in oxygen concentration. This continuous self-healing film of chloride covering the entire surface of the metal forms an effective protecting layer which prevents further attack at any point within the area covered by the coating.

In addition to the foregoing primary advantage of the employment of silver or the like in combination with the other elements of chromenickel stainless steel, we have found that other advantages of great importance result, as follows:

Increased thermal conductivity-Our novel alloy is found to have thermal conductivity which is considerably greater than that of ordinary 18-8 stainless. For instance, tests which we have made gave a heat conductivity of stainless steel of a composition corresponding to Formula 6, above, of .053 c. g. 5. units per second, whereas tests, made with the same apparatus and under the same conditions, of a standard 18-8 stainless steel without silver but otherwise of very nearly the same analysis, gave only .042 as. s. units per second. It is apparent that stainless alloys containing more than 0.14% of silver, would show even a greater increase in thermal conductivity. This increase of 26% or more in the heat conductivity of our improved alloy is a great advantage, since 18-8 stainless steel as now'made is unsatisfactory for use an many places where high heat conductivityis desirable.

"Less work hnrdening.We have found that our novel alloy is much less subject towork-hardening than ordinary 18-8 stainless steel, which hardens so much as to render machine operations such as filing and sawing extremely difilcult. 1K8 have found, for instance, that a piece of ordinary 18-8 stainless steel work-hardens so much after one or two strokes of a jewellers hack-saw that the saw will not cut it effectively afterwards, while a similar piece of metal of our novel alloy may be out completely through by the use of a singleblade. This property greatly increases the range of usefulness of stainless steel in the arts.

Free machining.-When turned in a lathe ordinary 18-8 stainless steel forms .a springy continuous chip which is likely to wind itself around the tool or the work and interfere with the machining operation. The alloy embodying our invention turns in about the same manner as ordinary cold rolled steel.

Surface polish-Steel embodying our invention can be given a better surface polish than comparable steels and this can be .done with much less difliculty. This property enhances the corrosive resistance of the metal, as polishing tends to render the surface of the material more uniform and therefore less susceptible to electrolytic action.

Toxic effect-4t is possible also that our alloy may have a toxic eflect and therefore tend to kill or discourage the growth of water organisms which might otherwise adhere to the surface of the metal and cause areas of difierent oxygen concentration.

While the exact proportions of the several ingredients in a stainless steel embodying our invention are susceptible of considerable variation, so long as the characteristics of the alloy as a typical stainless steel are not materially changed, it is believed that the most useful and practical proportion ranges of the several elements are substantially as follows: carbon, not more than 1%, chromium 6% to 20%, nickel and/or manganese 20% to 0.50%, and silver in effective amounts up to 1% in case of nickel and up to 2% in case of manganese, and to which may be added molybdenum up to 4%.

We claim:

1. The method of increasing theresistance of the 18 chromium-8 nickel type of stainless steel to pit corrosion in saline media which consists of forming on the exposed surface of the steel a continuous self-healing film of a silver compound originating from silver which is intra-granularly included in the steel.

2. The improved chromium-nickel stainless steel of the 18 chromium-8 nickel type, characterized by a marked resistance to pit corrosion in saline media, having carbon not over 0.35% and containing silver in eifective amounts up to 1%, the balance being iron with impurities in an amount which will not alter the properties of the alloy. 1

3. The improved stainless steel alloy characterized by a marked resistance to pit corrosion in saline media, containing chromium about 18%, nickel about 8%, carbon not over 0.35% and silver up to 0.50%,the balance being iron with impurities in an amount which will not alter the properties of the alloy.

4. The improved stainless steel characterized by a marked resistance to pit corrosion in saline media, containing between approximately 18% and 20% of chromium, nickel between approximately 8% and 12%, carbon not over 1%, and silver in effective amounts up to 1%, the balance being iron, with impurities in an amount which will not alter the properties of the steel.

5. The improved stainless steel alloy characterized by a marked resistance to pit corrosion in saline media, containing chromium not less than about 18% nor more than about 20%, not less than about 8% nor more than about 12% of a metal which forms a solid solution with silver and chosen from the group consisting of nickel and manganese, carbon not over 1% and silver in eiTective amounts up to 2%, the balance being iron with impurities in an amount which will not alter the properties of the alloy.

ALBERT L. KAYE.

ROBERT S. WILLIAMS. JOHN WULF'F. 

